Method of and apparatus for manufacturing semiconductor thin film, and method of manufacturing thin film transistor

ABSTRACT

A method of manufacturing a semiconductor thin film includes (A) forming an amorphous semiconductor film on a substrate, (B) irradiating a beam to a surface of the amorphous semiconductor film such that a predetermined region of the amorphous semiconductor film is melted and solidified to form a crystallized semiconductor film, and (C) scanning the beam in a first direction. A second direction is a direction on the surface of the amorphous semiconductor film perpendicular to the first direction. A length along the second direction of a cross section of the beam is substantially equal to or less than two times a width along the second direction of the crystallized semiconductor film.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/856,275 filed May 28, 2004 (pending), which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of manufacturing asemiconductor thin film, an apparatus for manufacturing a semiconductorthin film, and a method of manufacturing a thin film transistor (TFT).More particularly, the present invention relates to a method ofmanufacturing a semiconductor thin film in which the formation of agrain boundary is controlled, and a thin film transistor having a layerof the semiconductor thin film.

2. Description of the Related Art

As a display of an electronic product such as a personal computer andthe like, a liquid crystal display (LCD) is known and widely used. Inparticular, an active matrix liquid crystal display (AM-LCD) has beenrapidly popularized in recent years, because high quality images can beachieved due to switching devices provided for respective pixels. In theAM-LCD, a thin film transistor (referred to as “TFT”, hereinafter) isused as a switching device for controlling a pixel and as a driver IC.In addition to a liquid crystal display, such a TFT is also used in adriving-circuit-integrated-type image sensor and a fluorescent displaytube and the like.

FIGS. 1A to 1D show conventional processes to manufacture theabove-mentioned TFT. At first, as shown in FIG. 1A, an amorphous siliconfilm 202 is formed on a surface of a glass substrate 100, for example.This amorphous silicon film 202 is a precursor film of a semiconductorlayer which will be described later. Next, as shown in FIG. 1B, a laserlight 300 is irradiated to the surface of the amorphous silicon film202, and crystal grains are grown. As a result, a polycrystallinesilicon (poly-silicon) film 102 is formed from the amorphous siliconfilm 202. Here, the laser annealing is carried out by scanning the laserlight 300 from one end (for example, the left end in FIG. 1B) of theamorphous silicon film 202 to the other end (the right end in FIG. 1B).

Then, as shown in FIG. 1C, a gate insulating film 104 is formed on theformed poly-silicon film 102. Then, the channel-doping is performed on achannel region 120 of the semiconductor layer (poly-silicon film) 102.After that, a gate electrode 106 is formed on the gate insulating film104.

Next, as shown in FIG. 1D, a first inter-layer insulating film 130 isformed so as to cover the gate electrode 106 and the gate insulatingfilm 104. Then, contact holes 108 a, 110 a are formed to penetrate thefirst inter-layer insulating film 130 and the gate insulating film 104.Then, a source electrode 108 connected to the contact hole 108 a and adrain electrode 110 connected to the contact hole 110 a are formed onthe first inter-layer insulating film 130. Then, a second inter-layerinsulating film 132 is formed so as to cover the source electrode 108and the drain electrode 110. Thus, the TFT is manufactured.

In recent years, the fineness of an LCD tends to be increasinglyimproved, and also the higher performance is required for an LCD tosupport the higher resolution moving image. Therefore, a TFT used tocontrol the pixels is desired to operate faster. A TFT can operatefaster as the mobility of carrier (electron or hole) in the poly-siliconfilm 102 increases. However, if there are a large number of grainboundaries in the poly-silicon film 102, the mobility of the carrierdecreases, which results in a problem that the mobility of the TFT cannot be made faster.

Therefore, a technique has been proposed, in which the crystal growthduring the laser annealing process is controlled in order to reduce thenumber of the grain boundaries in the poly-silicon film 102 and hence toimprove the mobility of the carrier.

Japanese Laid Open Patent Application (JP-P-2002-217206) discloses atechnique in which a rectangular laser line beam is irradiated to form apoly-silicon layer which has crystal grains extending for a length equalto about half of a width of the laser line beam.

FIG. 2 is a schematic picture for explaining this technique. Accordingto this technique, the rectangular laser line beam with the size ofabout 5 μm×100 μm is irradiated to an amorphous silicon layer 303 shownin FIG. 2. At this time, the profile of the laser beam energy density istrapezoidal as shown on the left side of FIG. 2. As a result, siliconseed crystals (not shown) are randomly generated in portions of theamorphous silicon layer indicated by Y1 and Y2 which correspond to theends of the laser line beam. Then, poly-silicon grows from those siliconseed crystals toward a portion which corresponds to the center of thelaser line beam and is indicated by Y3. The growth of the poly-siliconstops at the portion Y3. In this way, a poly-silicon layer 303′ (331,332) is formed, which has crystal grains extending for the length equalto about half of the width of the laser line beam. Here, the grainboundaries extend from the portion Y1, Y2 to the portion Y3.

According to this technique, it is possible to make the mobility of thethin film transistor and ON-state current higher by setting the growthdirection of the crystal (poly-silicon) into alignment with a carrierrunning direction in the thin film transistor. However, there is aproblem with this technique in that the formation of the grainboundaries can not be controlled, even though the growth direction ofthe crystal can be controlled to some degree. If the grain boundarycrosses the channel portion of the thin film transistor, the desiredmobility of the carrier can not be obtained. Also, the diameter of thecrystal grain is at most the length equal to the half of the width ofthe laser line beam. Thus, there is also a problem with this techniquein that poly-silicon with a large grain diameter can not be formed.

As another technique, James S. Im et al. reports a technique in which anarrow beam is scanned to form a giant crystal grain in the scanningdirection (refer to a non-patent document: “Sequential lateralsolidification of thin silicon films on SiO2”, Robert S. Sposili andJames S. Im, Appl. Phys. Lett 69 (19) 1996 pp. 2864-2866).

FIGS. 3A to 3E schematically show the processes according to thistechnique. In this technique, the narrow beam 310 shown in FIG. 3A ismade from a pulse laser light by using an appropriate mask. The narrowbeam 310 has a width of 5 μm and a length of 200 μm, and is irradiatedto an amorphous silicon film 202. This narrow beam 310 is to be scannedalong the direction E shown in FIG. 3A, i.e., from one side (indicatedby a numeral D in FIG. 3A) to the opposite side (indicated by a numeralS in FIG. 3A). Thus, the amorphous silicon film 202 is to besequentially heated (annealed) as shown in FIGS. 3B to 3E.

FIG. 3B shows a situation when the first irradiation of the narrow beam310 is finished. At this time, the crystallization (solidification) ofthe melted amorphous silicon film 202 starts from boundaries between“the melted region” and “the non-melted region” (indicated by two-dotchain lines in FIG. 3B). Here, these boundaries correspond to the endsof the narrow beam 310 (top and bottom ends in FIG. 3B). Then, thegrowth of the crystals begins from those boundaries toward the center ofthe melted region. In this way, the solidified portion becomes acrystallized poly-silicon film 102. The growth of the crystals stopswhen they collide with each other near the center. Thus, a grainboundary B is formed near the center portion as shown in FIG. 3B. Itshould be noted that the crystallization advances in the lateraldirection in FIG. 3B, and a lot of grain boundaries are formed along thelongitudinal direction. These longitudinal grain boundaries will bedescribed later.

Next, as shown in FIG. 3C, the irradiation region (the narrow beam 310)is moved upward, and the second irradiation of the narrow beam 310 isperformed. Here, the displacement of the narrow beam 310 is 0.75 μm.

Similarly, the crystallization (solidification) starts from boundariesbetween “the melted region” and “the non-melted region” as shown in FIG.3D. These boundaries are indicated by two-dot chain lines in FIG. 3D,and correspond to the ends of the narrow beam 310. Crystals grow fromthose boundaries toward the center of the melted region, and a grainboundary B′ is formed near the center portion as shown in FIG. 3D. Atthis time, the grain boundary B formed in the previous step disappearsdue to the melt. Also, the melted portion along the lower boundarycrystallizes based on the crystal formed by the previous step (the firstirradiation). Therefore, no lateral grain boundary is formed along thislower boundary.

After that, the irradiation region (the narrow beam 310) is moved andthe irradiation of the narrow beam 310 is performed sequentially. Themelt and crystallization of the amorphous silicon film 202 are repeatedin the similar way. Thus, the crystal grains extending in thelongitudinal direction (the scanning direction E) can be formed as shownin FIG. 3E, and no grain boundary is formed along the lateral direction(orthogonal to the scanning direction E).

Also, Matsumura et al. reports a technique that uses a light-shieldplate in laser annealing (refer to a non-patent document:“Excimer-Laser-Induced Lateral-Growth of Silicon Thin Films”, KensukeIshikawa, Motohiro Ozawa, Chang-Ho Oh and Masakiyo Matsumra, Jpn. J.Appl. Phys. Vol. 37 (1998) pp. 731-736). FIG. 4A is a schematic picturefor explaining this technique, and FIG. 4B is a cross sectional viewalong a dashed line J-J′ in FIG. 4A.

According to this technique, a light-shield plate 400 is placed in alight path to shade a part of the narrow beam 300A, as shown in FIGS. 4Aand 4B. A part of the narrow beam 300A is diffracted to be a diffractedbeam 310A as shown in FIG. 4B, and the energy density of this diffractedbeam 310A becomes low. Therefore, the temperature of the amorphoussilicon film 202 associated with the diffracted beam 310A becomes lowerthan that associated with the directly incoming narrow beam 300A. Inother words, the temperature of the irradiation region on the left sideof FIG. 4B is higher than the temperature on the right side, i.e., atemperature gradient is generated as indicated by an arrow in FIG. 4B.This temperature gradient promotes the growth of the poly-silicon film102.

According to the conventional techniques mentioned above, the growthdirection of the crystal can be controlled to some degree. However, inthe technique disclosed in the above-mentioned patent document, theformation of the grain boundaries can not be controlled. There is also aproblem with the other conventional techniques in that the grainboundaries along the scanning direction can not be controlled.

FIG. 5 is a schematic picture explaining such a problem in theconventional technique. In FIG. 5, the longitudinal direction I denotesthe scanning direction. As shown in FIG. 5, crystals grow along thedirection I. However, each crystal can not grow largely in the directionorthogonal to the direction I, and a lot of grain boundaries B along thedirection I are generated in the poly-silicon film 102.

Here, the formation of these grain boundaries B is not controlled, i.e.,the locations and the distribution density of the grain boundaries B arenot controlled. Therefore, there is a certain distribution in thedistances between grain boundaries B adjacent to each other (eachdistance corresponds to a width t of each crystal 102 a). The maximumvalue of the distance is about 1 μm. Even if the movement direction ofthe carriers in the TFT is set to the direction I, the movement of thecarriers is interfered by the grain boundaries B which are bent in thelateral direction (orthogonal to the direction I) or intersect withother grain boundaries B. This causes the depression of the carriermobility. Moreover, the widths t of the respective crystals 102 a arenot constant and vary depending on the manufacturing condition.Furthermore, the number and the directions of the grain boundaries Balso vary. Thus, the carrier mobility and the threshold voltage of themanufactured TFT vary depending on the manufacturing condition.

The following may be considered as a reason for the formation of thecrystals 102 a with the various widths t. When the first irradiation ofthe laser light 300 is performed on the amorphous silicon film 202, thetemperature gradient is not generated along the direction orthogonal tothe direction I in the melted amorphous silicon film 202. Therefore,crystal seeds are formed randomly in the direction orthogonal to thedirection I. Then, the crystals grow based on the crystal seeds andextend along the direction I as mentioned above. As a result, variouscrystal grains with various widths t are formed as shown in FIG. 5.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodand an apparatus for manufacturing a semiconductor thin film which canreduce the number of grain boundaries in the manufactured semiconductorthin film.

Another object of the present invention is to provide a method and anapparatus for manufacturing a semiconductor thin film which can controlthe positions of grain boundaries.

Still another object of the present invention is to provide a thin filmtransistor in which a mobility is improved and variation of the mobilityis suppressed, and a method for manufacturing the thin film transistor.

In an aspect of the present invention, a method of manufacturing asemiconductor thin film includes (A) forming an amorphous semiconductorfilm on a substrate, (B) irradiating a beam to a surface of theamorphous semiconductor film such that a predetermined region of theamorphous semiconductor film is melted and solidified to form acrystallized semiconductor film, and (C) scanning the beam in a firstdirection.

The first direction is a scanning direction and a second direction is adirection on the surface of the amorphous semiconductor filmperpendicular to the first direction. A “beam length” is defined by alength along the second direction of a cross section of the beam. A“crystal growth width” is defined by a width along the second directionof the crystallized semiconductor film. The crystallized semiconductorfilm may include a single crystal region and a small poly-crystallinesilicon region formed around the single crystal region.

According to the present invention, the beam length is substantiallyequal to or less than two times the crystal growth width. The beamlength can be substantially equal to or less than the crystal growthwidth. Preferably, the beam length is equal to or less than 6 μm.

In another aspect of the present invention, a method of manufacturing asemiconductor thin film includes (AA) forming an amorphous semiconductorfilm on a substrate, (BB) irradiating a plurality of beams to a surfaceof the amorphous semiconductor film such that predetermined regions ofthe amorphous semiconductor film are melted and solidified to form aplurality of crystallized semiconductor films, and (CC) scanning theplurality of beams in the first direction. A “beam length” is a lengthalong the second direction of a cross section of each of the pluralityof beams. A “crystal growth width” is a width along the second directionof corresponding one of the plurality of crystallized semiconductorfilms.

According to the present invention, the beam length is substantiallyequal to or less than two times the crystal growth width. A plurality ofthe beam lengths of the plurality of beams can be substantially equal toor less than a plurality of the crystal growth widths of the pluralityof crystallized semiconductor films, respectively. Preferably, the beamlength of each beam is equal to or less than 6 μm.

In the present invention, the plurality of beams are arranged along thesecond direction. In this case, an interval between one of the pluralityof beams and an adjacent one of the plurality of beams is preferablyequal to or more than 0.3 μm. More preferably, the interval is equal toor more than 0.4 μm. More preferably, the interval is equal to or morethan 0.6 μm.

In the present invention, each of the plurality of beams belongs to anyone of a plurality of beam groups. Thus, each of the plurality of beamgroups includes a predetermined number of the beams. The plurality ofbeam groups are arranged in the first direction. The predeterminednumber of beams belonging to each beam group are arranged along thesecond direction.

Also, one of the plurality of beam groups includes a first beam. Anotherof the plurality of beam groups includes a second beam. A part of thefirst beam overlaps with a part of the second beam along the firstdirection. Preferably, a width for which the first beam overlaps withthe second beam is equal to or more than 0.7 μm.

In still another aspect of the present invention, a method ofmanufacturing a semiconductor thin film includes (D) forming anamorphous semiconductor film on a substrate, (E) irradiating a beam to asurface of the amorphous semiconductor film such that a predeterminedregion of the amorphous semiconductor film is melted and solidified toform a crystallized semiconductor film, and (F) scanning the beam in thefirst direction.

According to the present invention, during the (E) irradiating step, atemperature distribution of the surface along the second direction hastwo gradients near both ends of the predetermined region. Preferably,the temperature distribution along the second direction has atrapezoidal shape. A full width at half maximum (FWHM) of thetemperature distribution is substantially equal to or less than twotimes the crystal growth width. The FWHM of the temperature distributioncan be substantially equal to or less than the crystal growth width.Here, the two gradients along the second direction are generated bychanging amount of the beam along the second direction. Preferably, thechange in the amount of the beam along the second direction is equal toor more than 460.8 mJ/cm² per 1 μm on the surface.

In still another aspect of the present invention, an apparatus formanufacturing a semiconductor thin film has a laser oscillator composedfor generating a beam, a stage on which a substrate is placed, a mask,and a means for scanning the beam in a first direction. The mask isprovided on a light path of the beam and has a plurality of aperturesections through which the beam from the laser oscillator is irradiatedto a surface of the substrate.

The first direction is a scanning direction and a second direction is adirection on the surface of the substrate perpendicular to the firstdirection. A “beam length” is a length along the second direction of across section of the beam on the surface of the substrate. A “crystalgrowth width” is a width along the second direction of a crystal to beformed on the substrate.

According to the present invention, the beam length is substantiallyequal to or less than two times the crystal growth width. The beamlength can be substantially equal to or less than the crystal growthwidth. Preferably, the beam length is equal to or less than 6 μm.

In the apparatus, the plurality of aperture sections are arranged alongthe second direction.

Also, in the apparatus, each of the plurality of aperture sectionsbelongs to any one of a plurality of aperture groups. Thus, each of theplurality of aperture groups includes a predetermined number of theaperture sections. The plurality of aperture groups are arranged in thefirst direction. The predetermined number of aperture sections belongingto each aperture group are arranged along the second direction. Also,one of the plurality of aperture groups includes a first aperturesection, and another of the plurality of aperture groups includes asecond aperture section. A part of the first aperture section overlapswith a part of the second aperture section along the first direction.

In still another aspect of the present invention, a method ofmanufacturing a thin film transistor includes (a) forming an amorphoussemiconductor film on a substrate, (b) irradiating a beam to a surfaceof the amorphous semiconductor film such that a predetermined region ofthe amorphous semiconductor film is melted and solidified to form acrystal, (c) scanning the beam in the first direction to form acrystallized semiconductor thin film, and (d) forming a channel regionof the thin film transistor by using the crystallized semiconductor thinfilm such that carriers move along the first direction. During the (b)irradiating, the beam length is substantially equal to or less than twotimes the crystal growth width.

According to the method and the apparatus for manufacturing thesemiconductor thin film in the present invention, the following effectscan be attained. That is, the width of the crystal along the seconddirection can be made larger than that of a conventional semiconductorthin film, because of the temperature gradient generated along thesecond direction. Moreover, the positions of the grain boundaries can becontrolled, and also the number of the grain boundaries can be reduced.Also, the positions and the number of the crystals can be controlled.

Furthermore, according to the present invention, a thin film transistor(TFT) is manufactured by using the single crystal region generated bythe above apparatus and the above method. Thus, the mobility of themanufactured TFT is improved, because the number of the grain boundariesis reduced. This implies that a TFT with high mobility can be obtainedaccording to the present invention. Moreover, since the number and thedirections of the grain boundaries are controlled and hardly vary, thevariation in the mobility of the TFT can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1D show conventional processes to manufacture a thin filmtransistor;

FIG. 2 is a schematic view for explaining a conventional technique;

FIGS. 3A to 3E are schematic views for explaining another conventionaltechnique;

FIG. 4A is a schematic view for explaining still another conventionaltechnique;

FIG. 4B is a cross sectional view along a dashed line J-J′ in FIG. 4A;

FIG. 5 is a schematic view for explaining conventional techniques;

FIG. 6 shows a configuration of an apparatus for manufacturing asemiconductor thin film according an embodiment of the presentinvention;

FIG. 7 is a plan view showing an example of a mask in the apparatusaccording to the embodiment of the present invention;

FIG. 8 is a magnified sectional view showing a step of laser annealingaccording to the present invention;

FIG. 9 shows a distribution of laser energy density according to theembodiment of the present invention;

FIG. 10 is a plan view showing another example of a mask in theapparatus according to the present invention;

FIG. 11A is a plan view showing still another example of a mask in theapparatus according to the present invention;

FIG. 11B is a cross sectional view along a line G-G′ in FIG. 11A;

FIG. 12 is a schematic view showing a laser annealing apparatus used ina first example;

FIG. 13 is a cross sectional view schematically showing the structure ofa substrate to be processed according to the first example;

FIG. 14 is a cross sectional view schematically showing the structure ofa mask in the laser annealing apparatus according to the first example;

FIG. 15 is a plan view of the mask in the laser annealing apparatusaccording to the first example;

FIG. 16 shows a result of the SEM observation of the crystallized filmaccording to the first example;

FIG. 17 shows results of the SEM observations of the crystallized filmsaccording to a second example;

FIG. 18 is a schematic view showing the beam cross section according toa third example;

FIG. 19A shows a result of the SEM observation of the crystallized filmaccording to a comparative example;

FIG. 19B shows a result of the SEM observation of the crystallized filmaccording to the third example;

FIGS. 20A to 20C show results of the SEM observations of thecrystallized films according to Examples 4-1 to 4-3, respectively;

FIGS. 21A to 21C are schematic pictures showing the mechanism of thecrystal growth according to Examples 4-1 to 4-3, respectively;

FIGS. 22A and 22B show results of the SEM observations of thecrystallized films according to Examples 5-1 and 5-2, respectively;

FIG. 23 is a schematic picture showing an aperture pattern of a maskaccording to a sixth example;

FIGS. 24A and 24B show results of the SEM observations of thecrystallized films according to Examples 6-1 and 6-2, respectively;

FIG. 25A is a plan view schematically showing a semiconductor thin filmwith a single crystal region according to a seventh example; and

FIG. 25B is a plan view schematically showing a thin film transistoraccording to the seventh example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below withreference to the attached drawings.

FIG. 6 shows a configuration of a laser annealing apparatus, i.e., anapparatus for manufacturing a semiconductor thin film according to thepresent embodiment.

The laser annealing apparatus has a laser oscillator 60, a homogenizer62, a mirror 64, a mask 66, a lens 68, and a process chamber 70. Thelaser oscillator 60 is placed outside the process chamber 70. Theprocess chamber 70 has a window 72, a stage 74 and a mask 80. The window72 is provided on the wall surface of the process chamber 70. The stage74 and the mask 80 are provided within the process chamber 70. Asubstrate 90 with a precursor film 20A is placed on this stage 74.

A method of manufacturing a semiconductor thin film and an operation ofthe laser annealing apparatus are as follows. First, a precursor film20A is deposited on a substrate 90. This precursor film 20A is anamorphous semiconductor film or a poly-crystalline semiconductor film.For example, this precursor film 20A can be an amorphous silicon filmdeposited by a general deposition method such as a plasma CVD method andthe like. As shown in FIG. 6, the substrate 90 with the precursor film20A is placed on the stage 74 within the process chamber 70.

Next, an XeCl excimer laser with a wavelength of 308 nm is oscillated ina pulse shape by the laser oscillator 60. The laser light 30A outputtedfrom the laser oscillator 60 is changed to have a predetermined beamprofile through the homogenizer 62. Then, the laser light 30A isreflected by the mirror 64 to be incident into the mask 66. Passingthrough the mask 66, the shape of the laser light 30A is made into apredetermined beam shape. After adjusted by the lens 68, the laser light30A is incident into the process chamber 70 through the window 72.

The mask 80 is placed on the light path of the laser light 30A betweenthe window 72 and the precursor film 20A. This mask 80 is used forthrottling the laser light 30A, i.e., the “beam cross section” of theincoming laser light 30A is reduced by this mask 80. Here, the “beamcross section” refers to the area perpendicular to the beam axis. Then,the throttled laser light 30A is irradiated to the surface of theprecursor film 20A.

The laser light 30A is scanned over the precursor film 20A. Here, thestage 74 is designed to be movable in a “first direction”. The firstdirection is the scanning direction and is defined by a directionvertical to the paper surface in FIG. 6. A “second direction” is definedby a direction on the surface of the precursor film 20A andperpendicular to the first direction, i.e., a direction perpendicular tothe first direction and the beam axis. The second direction is alsoindicated by a direction F in FIG. 6. The substrate 90 having theprecursor film 20A moves in the first direction together with the stage74. Thus, the laser light 30A is scanned in the first (scanning)direction over the precursor film 20A.

FIG. 7 is a plan view showing an example of the mask 80 in the processchamber 70. As shown in FIG. 7, the mask 80 is made of an Al plate, anda rectangular aperture 82 is formed in the center of the Al plate. Aswill be described later, this aperture 82 is placed on the light path ofthe laser light 30A. The laser light 30A passed through the aperture 82is irradiated to the precursor film 20A. Thus, in the mask 80 shown inFIG. 7, the aperture 82 is associated with a light transmitting section86, and the other section is associated with a light shielding section(non-transmitting section) 84. Here, “aperture length” is defined by alength of the aperture 82 along the second direction (direction F).Also, “aperture width” is defined by a width of the aperture 82 alongthe first direction (scanning direction).

FIG. 8 is a magnified sectional view showing a step of laser annealing.As shown in FIG. 8, the laser light 30A is irradiated to the precursorfilm 20A through the aperture 82. Here, a length of the beam crosssection along the second direction (direction F) is larger than the“aperture length” above the mask 80. Therefore, the laser light 30Apassing through the aperture 82 is diffracted, and the diffracted laserlight 30A is irradiated to a predetermined area of the precursor film20A. Consequently, the precursor film 20A is melted in the predeterminedarea to which the diffracted laser light 30A is irradiated (thepredetermined area is referred to as an “irradiated area”). After theirradiation of the laser light 30A is stopped, the melted area issolidified, and hence a crystallized semiconductor layer is formed.

FIG. 9 shows the distribution of laser energy intensity, i.e., thetemperature distribution of the precursor film 20A in the irradiatedarea at the time of the laser irradiation. The temperature distributionis a distribution along the second direction. As shown in FIG. 9, thetemperature distribution has a trapezoidal shape. A high temperaturearea indicated by a numeral “H” shown in FIG. 9 corresponds to a hightemperature area “H” shown in FIG. 8. Similarly, low temperature areas“L” on both sides of the high temperature area “H” in FIG. 9 correspondto low temperature areas “L” shown in FIG. 8. Since the laser light 30Ais diffracted as shown in FIG. 8, temperature gradients are generated onboth sides of the high temperature area H at the center of theirradiated area. The distance between the areas with temperaturegradients is in an order of sub-micron to micron.

Here, “beam length” and “beam width” are defined on the basis of thecross section of the diffracted laser light 30A. The “beam width” isdefined by a width of the beam cross section along the first direction(scanning direction). The “beam length” is defined by a length of thebeam cross section along the second direction (direction F). Also, the“beam length” may be defined by a FWHM (Full Width at Half Maximum) ofthe temperature distribution shown in FIG. 9.

According to the present invention, at the time of crystallizing thesemiconductor film as mentioned above, the “beam length” is set to avalue equal to or less than about two times a width along the seconddirection of the crystal to be formed. As a result, single-crystalregions extending in the first direction (scanning direction) can beformed in the precursor film 20A, which will be shown later.

FIG. 10 is a plan view showing another example of the mask 80 in theprocess chamber 70. In this mask 80B, two rectangular apertures 82B areformed. These apertures 82B are associated with light transmittingsections 86B, and a section between two apertures 82B is associated withan interval section 88B. The other sections are associated with lightshielding section 84B.

Also, the material of the mask 80 is not limited to aluminum (Al). FIG.11A is a plan view showing still another example of the mask 80 in theprocess chamber 70, and FIG. 11B is a sectional view along a line G-G′in FIG. 11A. This mask 80 c has a substrate made of a transparentmaterial such as quartz and the like. Further, non-transparent patternis formed on a predetermined area of the substrate. For example, thenon-transparent pattern is formed by patterning a metal film made of,for example, aluminum, molybdenum, chrome, tungsten silicide andstainless alloy and so on. Also, a protective film such as a chromeoxide film can be laminated over the non-transparent pattern. Also, asingle-layer or a multi-layer dielectric film can be used to form thenon-transparent pattern. Thus, light transmitting areas 82C withrectangular shapes and a light shielding area 84C are formed in themask.80C as shown in FIGS. 11A and 11B. These light transmitting areascorrespond to apertures. A part of the laser light 30A transmits theselight transmitting areas 82C to be irradiated on the precursor film 20A.

Moreover, in the mask 80 mentioned above, the shape of the aperture (thelight transmitting area) 82 is not limited to rectangle. The shape canbe polygon such as triangle, hexagon, octagon and the like. Also, theshape can be circle, ellipse and the like. Moreover, the aperture 82 canhave a chevron shape. The step of laser annealing by using a mask 80with any of these apertures 82 is similar to that in the case mentionedabove. So, its explanation is omitted.

Also, a lot of slits with extremely thin widths can be formed in themask 80. Or, a lot of holes can be formed concentrated in the mask 80.These slits or holes are used instead of the aperture (the lighttransmitting area) 82. In this case, the energy of the laser light 30Acan be adjusted by changing the number of slits or the number anddensity of the holes.

Moreover, the above-mentioned mask 80 can be provided not only in theprocess chamber 70 as in FIG. 6 but also outside the process chamber 70.That is, the mask 80 can be placed at any position on the light pathfrom the laser oscillator 60 to the precursor film 20A. Also, the mask66 shown in FIG. 6 can be configured to have the non-transmission areasimilar to the mask 80 shown in FIGS. 7, 10, 11. In this case, the mask66 replaces the mask 80, and plays the same role as the mask 80.

Furthermore, the laser light 30A used in the laser annealing is notlimited to the XeCl excimer laser. Another excimer laser such as a KrFlaser can be used as the laser light 30A. Also, a solid laser such as anNd:YAG laser, an Nd:YLF laser, an Nd:YVO₄ laser and the like can beused. Further, a gas laser such as a carbonic acid gas laser, an argongas laser and the like can be used.

A TFT (Thin Film Transistor) can be formed on the surface of variousinsulating substrates by using the manufacturing method according to thepresent invention. In particular, it is preferable with regard to thehigh integration and the high performance to use an SOI (Silicon OnInsulator) in which an oxide film is formed on the surface of a siliconwafer.

Next, a plurality of examples of the manufacturing according to thepresent invention will be shown below together with experimentalresults.

FIRST EXAMPLE

FIG. 12 is a schematic picture showing a laser annealing apparatus (anapparatus for manufacturing a semiconductor thin film) used in thepresent example. The laser annealing apparatus has an excimer laseroscillator 60A, mirrors 152, 153, 154, a homogenizer 155, a mask stage157, a 1/3 reduction-imaging lens 160, and a process chamber 70. Thelaser annealing apparatus also has a mask 66 which is-supported movablyby the mask stage 157. The substrate stage 74 is provided within theprocess chamber 70, and a window 162 is provided on the surface of theprocess chamber 70.

According to this laser annealing apparatus, a substrate 165 is placedon the substrate stage 74. Then, a beam 150L (XeCl, wavelength of 308nm) is outputted from the excimer laser oscillator 60A. As shown in FIG.12, the beam 150L is injected into the process chamber 70 through themirrors 152, 153, the homogenizer 155, the mirror 154, the mask 66, the⅓ reduction-imaging lens 160 and the window 162. Then, the beam 150L isirradiated onto the substrate 165. Here, the mask stage 157 is composedof an XY stage, an air bearing, a linear motor and the like. Thus, thebeam 150L can be scanned with high precision by moving the mask stage157. It should be noted that the beam 150L can be scanned by moving thesubstrate stage 74, although it is scanned by moving the mask stage 157in the present laser annealing apparatus.

The cross section of the beam 150L is made into a shape corresponding toa pattern of the aperture of the mask 66. Then, the beam 150L isirradiated into a filed area. According to the present invention, a lenswith more aperture number (NA) than that of a usual laser annealingapparatus is used in order to make the intensity profile of the beam150L steep at laser edges. Here, the NA of the lens is set to 0.2. Also,the optical system is adjusted such that a length with which the energydensity of the beam 150L on the substrate 165 varies from 2% to 98% ofits maximum value is 1 μm.

Next, the substrate 165 will be described below. FIG. 13 is a crosssectional view schematically showing the structure of the substrate 165.As shown in FIG. 13, an SiO₂ film 170 is formed on a glass substrate 165a, and an amorphous silicon (a-Si) film 171 is formed on the SiO₂ film170 as the precursor film. In this example, no-alkali glass is used forthe glass substrate 165 a. More specifically, the 300 nm-SiO₂ film 170is deposited on the glass substrate 165 a by using an LP-CVD (LowPressure Chemical Vapor Deposition) method. The SiO₂ film 170 is forpreventing the diffusion of impurities from the glass. The a-Si film 171of 60 nm as the precursor film is formed on this SiO₂ film 170 by usingthe same LP-CVD method.

Next, the mask 66 will be described below. FIG. 14 is a cross sectionalview schematically showing the structure of the mask 66. In this mask66, a chrome film 175 is formed on a quartz substrate 166 a. The chromefilm 175 plays a role of the light shielding section (referred to as ashading section, hereinafter). Thus, a plurality of aperture sections175 a are formed as an aperture pattern. These aperture sectionscorrespond to light transmitting sections.

FIG. 15 is a plan view of the mask 66 showing the aperture pattern. Theaperture pattern has a plurality of unit patterns arranged repeatedly inone direction. In FIG. 15, one rectangular aperture 175 a is the unitpattern, and a plurality of the rectangular apertures 175 a are arrangedin x direction (the second direction mentioned above). As shown in FIG.15, the width of the aperture 175 a along the y (first) direction is “a”(“aperture width”), and the length of the aperture 175 a along the x(second) direction is “b” (“aperture length”). Here, the y (first)direction is consistent with the scanning direction. Also, the distancebetween the adjacent apertures 175 a is “c”, which is referred to as an“aperture interval”.

The laser annealing apparatus having the above-mentioned composition isused to manufacture the semiconductor thin film. The manufacturingprocess is carried out under the following experiment conditions shownin table 1.

TABLE 1 LASER FLUENCE 550 mJ/cm² APERTURE WIDTH a 12 μm APERTURE LENGTHb 6 μm APERTURE INTERVAL c 1.8 μm STEP WIDTH 0.2 μm

As mentioned above, the length with which the energy density of the beam150L on the substrate 165 varies from 2% to 98% of its maximum value(laser fluence: 550 mJ/cm²) is set to 1 μm. Therefore, the gradient ofthe energy density is given by 528 mJ/cm²/μm.

Also, the “aperture width” indicates the length “a” in FIG. 15, the“aperture length” indicates the length “b” in FIG. 15, and the “apertureinterval” indicates the length “c” in FIG. 15. The values a, b, cindicate lengths on the mask 66. Due to the ⅓ reduction-imaging lens 160(see FIG. 12), the “beam width” and the “beam length” on the substrate165 are one third of the “aperture width a” and the “aperture length b”,respectively. More specifically, the “beam width” is 4 μm and “the beamlength” is 2 μm on the a-Si film 171 of the substrate 165. The “stepwidth” in the table 1 indicates the distance for which the beam 150Lmoves between two pulse irradiation. The scanning direction (firstdirection) is indicated by an arrow in FIG. 15 (upward).

After manufacturing the semiconductor thin film, the Secco-etching isperformed on that crystallized film in order to emphasis grainboundaries. Then, the obtained crystallized film is observed by using ascanning electron microscope (SEM). FIG. 16 shows a result of the SEMobservation of the crystallized film formed under the conditions shownin the table 1.

It can be clearly seen from FIG. 16 that a single crystal region with nograin boundary is formed along the first (scanning) direction in thecenter of the image. Here, the image in FIG. 16 shows an areacorresponding to one aperture section 175 a and two shading sections,and it is found that the single crystal region is formed in an areacorresponding to the one aperture section 175 a. On the other hand,formed in areas corresponding to the two shading sections are groups ofgrain boundaries which extend so as to mark off the single crystalregion. Thus, it is confirmed that it is possible according to themanufacturing method of the present invention to form single crystalregions with controlling locations of the grain boundaries.

SECOND EXAMPLE

Another experiment is carried out as a second example, in which theaperture interval c is changed as a variable. The experimentalconditions other than the aperture interval are the same as those in thefirst example. In the present experiment, the aperture interval c on themask 66 is set to 0 μm, 0.6 μm, 0.9 μm, 1.05 μm, 1.2 μm and 1.8 μm.

FIG. 17 shows results of the SEM observations of the crystallized filmsformed under the conditions with respective aperture intervals. Theaperture intervals used are indicated at the bottom of respectiveimages, and the scanning direction (first direction) is indicated by anarrow. As is clearly seen from FIG. 17, the appearances of thecrystallized films vary with changing aperture intervals.

In the cases when the aperture interval is set to 0 μm or 0.6 μm, theinfluence of the shading section hardly appears in the SEM observation.In the case when the aperture interval is set to 0.9 μm, the differencein crystalline state begins to appear between the area corresponding tothe shading section and the area corresponding to the aperture section.In the case when the aperture interval is set to 1.2 μm, it is foundthat a single crystal region with no grain boundaries is formed underthe aperture 175 a. The width of the single crystal region along thesecond direction is about 1 μm. In the case when the aperture intervalis set to 1.8 μm (the same condition as in the first example), singlecrystal regions with no grain boundaries are formed under the aperture175 a with higher probability. The width of the single crystal regionalong the second direction is also about 1 μm.

THIRD EXAMPLE

Another experiment is carried out as a third example, in which the shapeof the beam shape is changed. The beam shape is rectangle as shown inFIG. 18. The beam width is a width of the beam cross section along thescanning direction (x direction in FIG. 18). The beam length is a lengthof the beam cross section along the second direction (y direction). Thelaser annealing apparatus similar to that in the first example is usedto manufacture the semiconductor thin film. The experiment conditionsaccording to the present example are shown in table 2.

TABLE 2 EXAMPLE COMPARATIVE 3 EXAMPLE LASER FLUENCE(mJ/cm²) 480 480 STEPWIDTH(μm) 0.2 0.2 SCAN DISTANCE(μm) 100 100 APERTURE WIDTH (μm) 21 9.9APERTURE LENGTH (μm) 12 270

In the present example (indicated by Example 3 in table 2), the laserfluence is 480 mJ/cm², and hence the gradient of the energy density isgiven by 460.8 mJ/cm²/μm. The step width is set to 0.2 μm, and the scanof the beam is carried out for 100 μm (scan distance). Also, theaperture width and the aperture length are set to 21 μm and 12 μm,respectively. Due to the lens 160 (see FIG. 12), the beam width and thebeam length on the substrate 165 are one third of the aperture width andthe aperture length, respectively. Thus, the beam width is 7 μm and thebeam length is 4 μm on the a-Si film 171 of the substrate 165.

For comparison, a comparative experiment is also carried out. In thecomparative experiment (indicated by Comparative Example in table 2),the laser fluence is 420 mJ/cm². The step width and the scan distanceare the same as those in the Example 3. The aperture width and theaperture length are set to 9.9 μm and 270 μm, respectively. Thus, thebeam length is 90 μm on the the a-Si film 171 of the substrate 165,which is much longer than that in the Example 3.

FIGS. 19A and 19B show results of the SEM observations of thecrystallized films according to the comparative example and the thirdexample, respectively. Here, after Secco-etching treatment, thecrystallized films are observed. In the comparison example, as shown inFIG. 19A, oblique grain boundaries are generated randomly in thecrystallized film.

On the contrary, in the third example where the aperture length is setto 12 μm and hence the beam length is set to 4 μm, as shown in FIG. 19B,a single crystal without random grain boundaries is formed in the centerof the irradiation region corresponding to the aperture section. Thesize of the single crystal is 100 μm in the first direction (scanningdirection) and 2.5 μm in the second direction. Also, a micropolycrystalline silicon region is formed on both sides of the singlecrystal region. The size of the poly-Si region is slightly less than 1μm in the second direction. Thus, in the third example, the “beamlength” (4 μm) is less than two times a “crystal growth width”. Here,the “crystal growth width” is defined as a length along the seconddirection of the region including the single crystal region and themicro poly-Si region. As described above, according to the manufacturingmethod in the present invention, it is possible to form a single crystalregion with no grain boundaries which extends in the first direction. Athin film transistor (TFT) can be made on the single crystal regionassociated with the irradiated region. Thus, it is possible to obtainthe TFT with high uniformity and high mobility, because no grainboundary exists in the channel region of the TFT. This can solve theproblems in the conventional techniques.

The reason why the single crystal region with no grain boundary isformed as shown in FIG. 19 is considered to be as follows. At first,immediately after the start of the beam scanning, crystal grains withthe width of 1 μm or less grow randomly in the first direction from theregions corresponding to the beam edges. After that, during theirradiation process, the temperature gradient along the second directionis generated around the edges of the beam 150L in the second direction.Due to this temperature gradient, one of the crystal grains grows notonly in the first direction but also preferentially in the seconddirection. As a result, the random oblique grain boundaries do notappear, and the single crystal region is formed whose size is about 2.5μm along the second direction.

FOURTH EXAMPLE

Another experiment is carried out as a fourth example in order toinvestigate the beam length dependence of the crystal growth, in whichthe beam length is changed as a variable. The laser annealing apparatusand the mask 66 similar to those in the first example are used tomanufacture the semiconductor thin film. The experiment conditionsaccording to the present example are shown in table 3.

TABLE 3 EXAM- EXAM- EXAM- PLE PLE PLE 4-1 4-2 4-3 LASER FLUENCE(mJ/cm²)480 480 480 STEP WIDTH(μm) 0.2 0.2 0.2 SCAN DISTANCE(μm) 100 100 100APERTURE WIDTH (μm) 21 21 21 APERTURE INTERVAL (μm) 3 3 3 APERTURELENGTH (μm) 12 18 30

In the present example, the laser fluence is 480 mJ/cm², and hence thegradient of the energy density is given by 460.8 mJ/cm²/μm. The stepwidth is set to 0.2 μm, and the scan distance is set to 100 μm. Also,the aperture width and the aperture interval are set to 21 μm and 3 μm,respectively. In Example 4-1, the aperture length is set to 12 μm, andhence the beam length is set to 4 μm. In Example 4-2, the aperturelength is set to 18 μm, and hence the beam length is set to 6 μm. InExample 4-3, the aperture length is set to 30 μm, and hence the beamlength is set to 10 μm. Under these conditions, the beam 150L isirradiated to the a-Si film 171.

FIGS. 20A to 20C show results of the SEM observations of thecrystallized films according to the Examples 4-1 to 4-3, respectively.Here, after Secco-etching treatment, the crystallized films areobserved. In each figure, images at the locations where the scanningstarts (Starting Location) and ends (Ending Location) are shown. Also,FIGS. 21A to 21C are schematic pictures showing the mechanism of thecrystal growth in the semiconductor thin film according to the Examples4-1 to 4-3, respectively.

As shown in FIG. 20A, when the aperture length is set to 12 μm and thebeam length is set to 4 μm, single crystal regions are formed at thecenters of the irradiation regions. The reason for this result will beas follows. As shown in FIG. 21A, the temperature gradient along thesecond direction (direction y in FIG. 20A) is generated, whichsuppresses the occurrence of oblique grain boundaries as explainedabove. Thus, the single crystal regions are formed at the centers of theirradiation regions.

As shown in FIG. 20B, when the aperture length is set to 18 μm and thebeam length is set to 6 μm, one straight grain boundary extending in thefirst direction (direction x) is formed only at the center of theirradiation region. The reason for this result will be as follows. Asshown in FIG. 21B, the temperature gradient along the second direction(direction y in FIG. 20B) is generated, which suppresses the occurrenceof oblique grain boundaries. Also, the “beam length” is consistent withtwo times the “crystal growth width” which is the length along thesecond direction of the region including the single crystal region andthe small poly-Si region.

As shown in FIG. 20C, when the aperture length is set to 30 μm and thebeam length is set to 10 μm, grain boundaries extending in the firstdirection (direction x) are formed at the center of the irradiationregion. The reason for this result will be as follows. As shown in FIG.21C, the temperature gradient along the second direction (direction y inFIG. 20C) around the beam edge suppresses the occurrence of obliquegrain boundaries to some extent. However, since the “beam length”becomes longer than two times the “crystal growth width”, the effect ofthe temperature gradient along the second direction becomes scarce.That's why oblique grain boundaries are formed randomly around thecenter of the irradiation region.

It is obvious from the results mentioned above that the single crystalregion can be formed in the a-Si film 171 with controlling the positionsof grain boundaries when the beam length is set to less than 6 μm, orpreferably less than 4 μm. Thus, it is possible to manufacture the TFTwhich has high mobility and high uniformity. It should be noted that thepreferable beam length depends on thickness and a deposition method ofthe a-Si film 171 as the precursor film, laser fluence of the beam 150L,the resolution of the optical system and so on. Hence, the beam lengthis not limited to 6 μm or 4 μm, and can be set to an appropriate valueaccording to manufacturing conditions. According to the presentinvention for manufacturing the semiconductor thin film, the “beamlength” (4 μm) is set to a value equal to or less than two times the“crystal growth width”. Here, the “crystal growth width” indicates alength along the second direction of the region including the singlecrystal region and the small poly-Si region.

FIFTH EXAMPLE

Another experiment is carried out as a fifth example in order toinvestigate the aperture interval (beam interval) dependence of thecrystal growth, in which the aperture interval is changed as a variable.The laser annealing apparatus and the mask 66 similar to those in thefirst example are used to manufacture the semiconductor thin film. Theexperiment conditions according to the present example are shown intable 4.

TABLE 4 EXAMPLE 5-1 EXAMPLE 5-2 LASER FLUENCE(mJ/cm²) 480 480 STEPWIDTH(μm) 0.2 0.2 SCAN DISTANCE(μm) 100 100 APERTURE WIDTH (μm) 21 21APERTURE LENGTH (μm) 12 12 APERTURE INTERVAL (μm) 30 1.5

In the present example, the laser fluence is 480 mJ/cm², and hence thegradient of the energy density is given by 460.8 mJ/cm²/μm. The stepwidth is set to 0.2 μm, and the scan distance is set to 100 μm. Also,the aperture width and the aperture length are set to 21 μm and 12 μm,respectively. Thus, the beam length is set to 4 μm. In Example 5-1, theaperture interval is set to 30 μm, and hence the beam interval is set to10 μm. In Example 5-2, the aperture interval is set to 1.5 μm, and hencethe beam interval is set to 0.5 μm. Under these conditions, the beam150L is irradiated to the a-Si film 171.

FIGS. 22A and 22B show results of the SEM observations of thecrystallized films according to the Examples 5-1 and 5-2, respectively.Here, after Secco-etching treatment, the crystallized films areobserved. It is found by comparing images in FIGS. 22A and 22B thatthere exists no conspicuous difference in size of the single crystalregion and crystalline status between the crystallized films formedunder both conditions. Thus, it is concluded based on the resultsobtained from the present example and the second example (see FIG. 17)that a certain length is necessary for the aperture interval. However,the aperture interval has no influence on the crystalline status of thecrystallized film formed on the a-Si film 171, if the aperture intervalis longer than the certain length.

SIXTH EXAMPLE

In the above-mentioned examples, the mask 66 shown in FIG. 15 is usedfor manufacturing the semiconductor thin film. In this mask 66 shown inFIG. 15, the plurality of the rectangular apertures 175 a are arrangedin the second direction.

Another experiment is carried out as a sixth example in order to reducethe number of grain boundaries. Used in the present example formanufacturing the semiconductor thin film is a laser annealing apparatuswhich is similar to that in the first example but has a different mask66. FIG. 23 is a schematic picture showing an aperture pattern of themask 66 in the sixth example. The mask 66 according to the presentexample has a plurality of aperture groups arranged in the firstdirection (scanning direction; direction x). Each of the plurality ofaperture-groups includes a plurality of rectangular apertures arrangedalong the second direction (direction y). For example, the mask 66 inFIG. 23 has two aperture groups, a front aperture group and a rearaperture group. The front aperture group includes three front aperturesarranged along the second direction. The rear aperture group includestwo rear apertures arranged along the second direction.

The “aperture width”, the “aperture length” and the “aperture interval”can-be defined in a similar way to the above-mentioned examples. A“front-rear interval” is defined by an interval between adjacentaperture groups, i.e., between the front aperture group and the rearaperture group. Also, as shown in FIG. 23, an “overlap width” is definedby a width along the second direction for which apertures belonging toadjacent aperture groups overlap.

The mask 66 mentioned above is used to manufacture the semiconductorthin film. The experiment conditions according to the present exampleare shown in table 5.

TABLE 5 EXAMPLE EXAMPLE 6-1 6-2 LASER FLUENCE(mJ/cm²) 480 480 STEPWIDTH(μm) 0.2 0.2 SCAN DISTANCE(μm) 100 100 APERTURE WIDTH (FRONT)(μm) 99 APERTURE LENGTH (FRONT)(μm) 12 12 APERTURE INTERVAL (FRONT)(μm) 3 6APERTURE WIDTH (REAR)(μm) 9 9 APERTURE LENGTH (REAR)(μm) 12 9 APERTUREINTERVAL (REAR)(μm) 3 9 FRONT-REAR INTERVAL(μm) 15 15 OVERLAP WIDTH(μm)4.5 1.5

In the present example, the laser fluence is 480 mJ/cm², and hence thegradient of the energy density is given by 460.8 mJ/cm²/μm. The stepwidth is set to 0.2 μm, and the scan distance is set to 100 μm. InExample 6-1, the aperture width, the aperture length and the apertureinterval for each aperture are set to 9 μm, 12 μm and 3 μm,respectively. Also, the front-rear interval and the overlap width areset to 15 μm and 4.5 μm, respectively. In Example 6-2, the aperturewidth, the aperture length and the aperture interval for each of frontapertures are set to 9 μm, 12 μm and 6 μm, respectively. The aperturewidth, the aperture length and the aperture interval for each of frontapertures are set to 9 μm, 9 μm and 9 μm, respectively. Also, thefront-rear interval and the overlap width are set to 15 μm and 1.5 μm,respectively. Under these conditions, the beam 150L is irradiated to thea-Si film 171. Here, the beam 150L corresponding to the front apertureis referred to as a front beam. Also, the beam 150L corresponding to therear aperture is referred to as a rear beam. Thus, a plurality of thefront beams belong to a “front beam group”, and a plurality of the rearbeams belong to a rear “ream group”.

FIGS. 24A and 24B show results of the SEM observations of thecrystallized films according to the Examples 6-1 and 6-2, respectively.Here, after Secco-etching treatment, the crystallized films areobserved. In each figure, images at the locations where the scanningstarts (Starting Location) and ends (Ending Location) are shown.

In the Example 6-1, the overlap width is 4.5 μm which means that anoverlap width of the beam 150L on the substrate (referred to as a “beamoverlap width”) is 1.5 μm. In this case, as shown in FIG. 24A, the smallpoly-Si regions formed by the front beams and regions corresponding tothe shading sections between the front apertures become single crystalregions because the irradiation of the rear beams is performed. Found inFIG. 24A are straight grain boundaries, which are formed along the firstdirection and are parallel with each other. Obviously, the positions ofthese grain boundaries are controlled, and the number of the grainboundaries are reduced. A TFT can be manufactured by using the obtainedsemiconductor film. In the TFT, the channel layer is formed such thatelectrons move along the first direction (a direction along which thesingle crystal regions are extending). Thus, it is possible to form theTFT having high mobility and high uniformity.

In the Example 6-2, the overlap width is 1.5 μm which means that anoverlap width of the beam 150L on the substrate (a beam overlap width)is 0.5 μm. In this case, as shown in FIG. 24B, the small poly-Si regionsremain between the single crystal regions extending in the firstdirection and formed by the front beams and the rear beams.

It is found from the results mentioned above that the small poly-Siregion extending in the first direction can be removed by setting thebeam overlap width to a value equal to or larger than the width alongthe second direction of the small poly-Si region. In this example, thebeam overlap width should be equal to or more than 0.7 μm. It should benoted that the number of aperture groups is not limited to two, and canbe more than three.

SEVENTH EXAMPLE

In the present example, the mobility of two TFTs will be compared. Afirst TFT was made from a semiconductor thin film formed by using a maskwith comparatively large aperture 175 a. A second TFT was made from asemiconductor thin film formed by using a mask with comparatively smallaperture 175 a.

In manufacturing the first TFT, the laser annealing apparatus and thesubstrate 165 similar to those in the first example were used. Here, the“aperture length” was set to 270 μm, and hence the “beam length” was setto 90 μm, which did not meet the requirements of the present invention.Also, the “aperture width” was set to 9.9 μm, and hence the “beam width”was set to 3.3 μm. The scan distance was 300 μm. As a result, apoly-crystalline film with oblique grain boundaries was formed underthose conditions. Then, the first TFT (n-ch TFT) was manufactured byusing this poly-crystalline film as an active layer. The channel lengthof the first TFT was 1.4 μm and the channel width of the first TFT was1.4 μm. The measured mobility of the first TFT obtained was 360 cm²/Vs.

In manufacturing the second TFT, the laser annealing apparatus and thesubstrate 165 similar to those in the first example were used. Here, the“aperture length” was set to 12 μm, and hence the “beam length” was setto 4 μm, which met the requirements of the present invention. Also, the“aperture width” was set to 21 μm, and hence the “beam width” was set to7 μm. The scan distance was 300 μm. As a result, a semiconductor thinfilm with a single crystal region was formed under those conditions.

FIG. 25A is a plan view schematically showing the semiconductor thinfilm with the single crystal region according to the present example. Asshown in FIG. 25A, the single crystal region 180 was formed in an a-Siregion 181. The length of the single crystal region 180 along thescanning direction was 300 μm corresponding to the scan distance. Also,the width of the single crystal region 180 along the second directionwas 2.5 μm. Then, an island 182 was formed in the single crystal region180 as shown in FIG. 25A. The length of the island 182 along thescanning direction (indicated by a numeral 1 in FIG. 25A) was 10 μm.Also, the width of the island 182 along the second direction (indicatedby a numeral w in FIG. 25A) was 1.4 μm.

Then, as shown in FIG. 25B, a source 187 and a drain 188 were formed onthe island 182. Further, a gate electrode 185 crossing over the island182 was formed through an insulating film (not shown). Also, contacts186 were formed so as to be electrically connected to the source 187 andthe drain 188, respectively. Thus, a field effect transistor is formed.Here, a channel region is formed such that carriers move along the firstdirection. In this way, the second TFT (n-ch TFT) was formed. Thechannel length of the second TFT was 1.4 μm and the channel width of thesecond TFT was 1.4 μm. The measured mobility of the second TFT obtainedwas 520 cm²/Vs.

It is obvious from the above comparison that the mobility of the secondTFT is higher than that of the first TFT. That is to say, the second TFTmanufactured by the method according to the present invention has highermobility than the first TFT. Thus, a TFT with high performance can beprovided by using the manufacturing method according to the presentinvention.

As described above in detail, according to the method and the apparatusfor manufacturing the semiconductor thin film in the present invention,the following effects can be attained. That is, the width of the crystalalong the second direction can be made larger than that of aconventional semiconductor thin film, because of the temperaturegradient generated along the second direction. Moreover, the positionsof the grain boundaries can be controlled, and also the number of thegrain boundaries can be reduced. Also, the positions and the number ofthe crystals can be controlled.

Furthermore, according to the present invention, a thin film transistor(TFT) is manufactured by using the single crystal region generated bythe above apparatus and the above method. Thus, the mobility of themanufactured TFT is improved, because the number of the grain boundariesis reduced. This implies that a TFT with high mobility can be obtainedaccording to the present invention. Moreover, since the number and thedirections of the grain boundaries are controlled and hardly vary, thevariation in the mobility of the TFT can be suppressed.

It will be obvious to one skilled in the art that the present inventionmay be practiced in other embodiments that depart from theabove-described specific details. The scope of the present invention,therefore, should be determined by the following claims.

1-18. (canceled)
 19. An apparatus for manufacturing a semiconductor thinfilm comprising: a laser oscillator configured for generating a beam; astage on which a substrate is placed; a mask provided on a light path ofsaid beam and having a plurality of aperture sections through which saidbeam from said laser oscillator is irradiated to a surface of saidsubstrate; a means for scanning said beam in a first direction; whereina second direction is a direction on said surface of said substrateperpendicular to said first direction, wherein a length along saidsecond direction of a cross section of said beam on said surface, as abeam length, is substantially equal to or less than two times a widthalong said second direction of a crystal to be formed on said substrate,as a crystal growth width.
 20. The apparatus according to claim 19,wherein said beam length is substantially equal to or less than saidcrystal growth.
 21. The apparatus according to claim 19, wherein saidplurality of aperture sections are arranged along said second direction.22. The apparatus according to claim 19, wherein each of said pluralityof aperture sections belongs to any one of a plurality of aperturegroups, and each of said plurality of aperture groups includes apredetermined number of said aperture sections, wherein said pluralityof aperture groups are arranged in said first direction, wherein saidpredetermined number of aperture sections belonging to said eachaperture group are arranged along said second direction.
 23. Theapparatus according to claim 22, wherein one of said plurality ofaperture groups includes a first aperture section, and another of saidplurality of aperture groups includes a second aperture section, whereina part of said first aperture section overlaps with a part of saidsecond aperture section along said first direction.
 24. (canceled)